IR ACTIVE PHONON-SPECTRA OF B-C-AL COMPOUNDS WITH BORON-CARBIDE STRUCTURE

The IR spectra of boron carbide containing up to about 1.5 at.% alumin um atoms are determined between 80 and 5000 cm(-1) for temperatures be tween 70 and 450 K, While the essential features of the boron carbide phonon spectrum remain largely unchanged, there are several new low en ergy absorption bands at about 190 and 290 cm(-1) obviously induced by the aluminum atoms. The band at 190 cm(-1) is attributed to the bendi ng mode of Al-2 chains, which are assumed to be formed in otherwise ch ain-free unit cells of boron-rich boron carbide, The dynamical conduct ivity of aluminum-doped boron carbide decreases with increasing alumin um content, while the Seebeck coefficient showing p-type behavior for ail compounds increases. This makes evident that doping with aluminum atoms changes the electronic properties of boron carbide. (C) 1997 Aca demic Press.